In South Florida, the coordinated walkouts began in the morning, where at least 25 high schools and colleges from Wellington to Miami honored the 17 teenagers and teachers killed in the Parkland massacre. Students and their allies brandished protest signs, with the goal of pressing lawmakers for stricter gun-control laws.

At Marjory Stoneman Douglas High School in Parkland, students assembled by the hundreds onto the football field, and police officers erected barricades along Holmberg Road to better guide the throng of students leaving campus.

"We're with you," yelled Michelle Pomerantz, 58, of Parkland, who was among hundreds of supporters applauding and cheering as they lined up along Holmberg Road as the students waited in the football field.

Historians said the demonstrations were shaping up to be one of the largest youth protests in decades.

“It seems like it's going to be the biggest youth-oriented and youth-organized protest movements going back decades, to the early ’70s at least,” said David Farber, a University of Kansas history professor who has studied social change movements. “Young people are that social-media generation, and it's easy to mobilize them in way that it probably hadn't been even 10 years ago.”

Back on the football field, the original song “Shine,” composed by Stoneman Douglas Drama Club students, began to play.

The song prompted supporters outside the school to cry. At 10:15 a.m., the students’ planned 17-minute rally transformed into a nearly 2-mile march to Pine Trails Park. As teenagers walked off campus, cars parked along the grass outside on the northbound lane honked in support.

The rallies came less than a week after Florida Republican Gov. Rick Scott, citing the students' actions, signed into law a bill that raises the minimum age for the purchase of long guns, including assault-style weapons such as the AR-15.

The bill also extends the three-day waiting period for handguns to long guns, and creates a program to enable some teachers or other school employees to carry guns.

The bill was a sign of progress, but it doesn’t go far enough, said Stoneman Douglas junior Susana Matta Valdivieso.

“We are here to protest because we know that more can be done, not just statewide but nationwide,” said Valdivieso, 17, one of Wednesday’s walkout organizers. “It's been a whole month and we're still out here protesting.”

Radio transmissions reveal the moment Coconut Creek police apprehended Nikolas Cruz after the school shooting at Marjory Stoneman Douglas High School on Feb. 14.

Radio transmissions reveal the moment Coconut Creek police apprehended Nikolas Cruz after the school shooting at Marjory Stoneman Douglas High School on Feb. 14.

Holding a microphone, 17-year-old Leonor Munoz, a senior at Stoneman Douglas, told thousands of students assembled at Pine Trails Park that they were marching “for those who will never be here again, but we’re fighting for those who might be next.”

“Our government has done nothing. We should all be in school,” Munoz said, and allowed herself a joke. “We should be studying for tests that we will probably fail. But we face a test of the future.”

Munoz appeared on the verge of tears as she concluded, shouting, “Never Again!” three times into the microphone.

As Stoneman Douglas students marched, so did thousands more in Washington, D.C.

Students gathered on Pennsylvania Avenue in front of the White House, hoisting colorful signs and cheering in support of gun control.

The students chanted, “Hey, hey, ho, ho. The NRA has got to go!” and “What do we want? Gun control! When do we want it? Now!”

Max Poteat, a student who helped organize a walkout at North Carolina's East Chapel Hill High School, said he was struck by the emotional weight of the moment.

“I think halfway through it really hit me, and I think everyone around, that these are teenagers just like us and that their lives were taken innocently — and that time is needed for change,” he said.

The idea behind National Walkout Day started with Empower, the youth branch of the annual Women’s March, who last month called for a 17-minute walkout, one minute for each of the 17 people killed in the shooting, and for protesters to wear orange.

But some of the walkouts lasted hours, not minutes.

In South Florida, students created symbols to try to represent the tragedy. At Cooper City High, students gathered around 14 empty desks and three podiums arranged in a circle outside the school, representing the 14 students and three faculty members killed in the shooting. The students then released 17 doves from a box.

In Parkland, students from Westglades Middle School, which is adjacent to Stoneman Douglas, also walked along Holmberg Road to participate in the day’s demonstrations.

There appeared to be early confusion among some students, who were unsure of the walkout destination.

One 13-year-old student on the phone, Landon Reuter, asked his dad for permission not to go back to school because “there are thousands of kids walking out.”

“No way I’m going back to school,” Reuter said. “Is that OK?”

Asked if school administrators are preventing students from returning to campus, Tracy Clark, a Broward County Schools district spokeswoman, said students who walked off campus should return as soon as possible.

“In order for students to be allowed back on campus, they need to actually go back to campus first,” Clark said.

Stoneman Douglas senior David Hogg, one of the largest student-activist voices advocating for gun control, live-streamed the walkout at the school on his YouTube channel.

“Every one of these individuals could have died that day,” Hogg said. “I could have died that day.”

While some listened to impassioned anti-gun violence speeches from fellow students, others huddled around Pine Trail Park’s makeshift memorials, where 17 crosses and Stars of David were ringed by flowers and handwritten notes.

The mood became somber as students, heads bent, cried and prayed around these vigils. A dozen girls sat around one for Meadow Pollack, a senior who was slain. Others sobbed at the memorial for Aaron Feis, a popular football coach.

Wayne Roustan / Sun-Sentinel

Three students deliver a speech outside the K.C. Wright Administration Building in downtown Fort Lauderdale on Wednesday, March 14. The rally was one of many protests held on National Walkout Day.

Three students deliver a speech outside the K.C. Wright Administration Building in downtown Fort Lauderdale on Wednesday, March 14. The rally was one of many protests held on National Walkout Day. (Wayne Roustan / Sun-Sentinel)

In Wellington, students at Palm Beach Central High School assembled in the school’s stadium to honor Parkland victims, according to Sun Sentinel news partner WPEC-CBS 12. Photos of the 17 slain teenagers and staff were placed on empty desks as students read brief descriptions of their lives.

Responding to Wednesday’s student protests, Broward Schools Superintendent Robert W. Runcie said the walkouts were planned solely by students, not the district.

“I haven’t seen a movement like this before, period,” Runcie said. “I certainly don’t encourage any student to be out of school, but you do have moments where they want to exercise their rights. Hats off to our young people.”

As Runcie spoke, about 300 students from eight Broward schools descended on the K.C. Wright Administration Building in downtown Fort Lauderdale, and took turns making speeches to anyone within earshot outside the Broward School district headquarters.

Wayne Roustan / Sun-Sentinel

A student holds up her protest sign in downtown Fort Lauderdale, where she protested gun violence on National Walkout Day.

A student holds up her protest sign in downtown Fort Lauderdale, where she protested gun violence on National Walkout Day. (Wayne Roustan / Sun-Sentinel)

Students representing schools from South Broward High School in Hollywood to Blanche Ely in Pompano Beach brandished hand-painted posters bearing messages such as “With rights come responsibilities. Stop gun violence.”

Nina Zamora, a Fort Lauderdale High School senior, said she wants stricter gun control, along with deeper background checks and banning AR-15s.

“The best-case scenario is the banning of [AR-15s] because they are really meant for protecting our country, not protecting ourselves," said Zamora, 17, a rally organizer.

By 4 p.m., the student protesters hopped Sun Trolley shuttles, which drove them back to their schools.

Other protests planned in coming weeks include the March for Our Lives rally for school safety, which organizers say is expected to draw hundreds of thousands to the nation's capital on March 24. Another round of school walkouts is planned for April 20, the 19th anniversary of the Columbine High shooting in Colorado.

Information from The Associated Press was used to supplement this news article.

YouTube livestream from David Hogg during the Marjory Stoneman Douglas walkout.

YouTube livestream from David Hogg during the Marjory Stoneman Douglas walkout.